This invention relates to an improved hook for hook and loop fasteners and particularly to plastic molded hooks intended for use with low pile loops. The technology of hook and loop fasteners is well known wherein a fastener comprised of two separable pile fastening tapes have interengaging piles on their surfaces, one pile have loop-elements and the other hook elements, are capable of co-acting to form a separable bond.
Such pile fasteners have found a wide variety of uses where ease of opening and closing is desirable such as in clothing, footwear, home furnishings, medical products, automotive fastening and many other industrial situations where detachable or permanent engagement is required. U.S. Pat. No. 3,009,235, U.S. Pat. No. 3,083,737 and U.S. Pat. No. 3,154,837 disclose various forms of separable pile fastener tapes constructed from fibrous forms of synthetic polymers such as nylon using basic textile weaving techniques. Such methods create a base fabric into which is woven the pile surface capable of engaging to form the closure. In more recent times special hook materials have been made from plastic molding techniques wherein the hooks are integrally formed with a base strip as the tape is being formed.
U.S. Pat. No. 3,031,730 describes a closure wherein a surface of burr like elements are exposed on a surface to be positively coupled with a fabric. The burr like elements are in the form of cast or molded flexible or plastic hook like members.
U.S. Pat. No. 3,760,000 to Menzin discloses a hook xe2x80x9ceyexe2x80x9d having a sloping surface which functions as a cam surface for extracting the molded hook from its mold cavity. The shank surface has two flat sides of equal dimensions and a somewhat larger third side. The shank portion is larger in cross section nearer the web than at the tip of the hook and the three flat side portions of the shank are continuous in smooth curves into and throughout the hook portion with the shank portion of the three sides laying in the same continuous plane as the corresponding face of the hook portion. U.S. Pat. No. 3,312,583 to Rochlis and U.S. Pat. No. 3,708,833 to Ribich describe other embodiments of hooks having somewhat tapered shapes. U.S. Pat. No. 3,913,183 to Brumlik describes a self gripping device wherein the gripping elements are particularly adapted for self gripping fibers and the like along the entire length of the fibers.
U.S. Pat. No. 4,894,060 to Nestegard describes a hook design for a disposable diaper with an improved hook fastener portion wherein the hook is made by the technique of extruding a profile and subsequently slitting the profile to form discrete hooks. The Nestegard patent claims a hook of sufficiently small dimensions for engaging with low cost loops, particularly loops created by the nonwoven process. The hook shape of the Nestegard patent is considerably different than those of the instant invention because of the method of making the hooks wherein one is dependent upon a continuous profile prior to the cross cutting process. The dimensions disclosed and claimed in the Nestegard patent are not sufficient to calculate a displacement volume.
Even more recently U.S. Pat. No. 4,984,339 to the inventors of the instant application discloses an improved hook having a profile defined by an inner, smoothly contoured, generally concave face and an outer, generally convex face, wherein the hook tapers smoothly and continuously downward in width from a sturdy base member to its free end whereby the hook will not deform to release a loop engaging the hook in shear at or below the desired applied force.
While the hooks formed according to these patents posses many useful properties and engage with a wide range of loop constructions, they possess the limitations of many other prior art hooks in their inability to function effectively with very low profile loops constructed with very short individual loops. Such loops are especially desirable because of their thinness and their low cost. In some case such loops are laminated to thin layers of polyurethane foam to provide a resilient base so that hooks can more easily penetrate into the body of a pile and thus be more easily surrounded by loops. In general, however, such loops do not function well with conventional hook structures.
One exception to the above described phenomena is the so-called mushroom hook. Mushroom hooks are produced by a variety of processes. Details of these types of products are contained in U.S. Pat. Nos. 3,138,841, 3,770,359, 4,024,003 and 4,290,832. Generally, the steps include creating an upstanding filament of polypropylene monofilament and melting the top of the monofilament with heat which causes molten polymer to xe2x80x9cmelt backxe2x80x9d or flow down the stem in a blob which solidifies at the terminal end of the filament to form a mushroom shape head on top of the stem. The mushroom head acts as do hooks of conventional hook and loop fasteners by entangling with loops to form a bond. Because of its small footprint, which will be discussed more fully below, mushroom fasteners are able to engage readily with lower pile loops than other hooks of the hook and loop type. However, mushroom products have many disadvantages. They are limited to use of orientated polypropylene fibers with associated limitations of that material, such as a relatively low temperature operating range. The mushroom heads are easily snapped off their stems giving such products very limited life in use, and the mushroom head does not have the flexing capability of a hook shape and therefore the only way a loop can be removed from the head is to rupture either the loop or the mushroom head. Other limitations of mushroom products are well known to those in the art.
The present invention contemplates producing a hook from the method described in U.S. Pat. No. 4,794,028 to Fischer in which both the size and shape of the hook is especially suited to low level loops. It has been found that outstanding and unexpected performance from such hooks in low level loops is possible. It is further realized that the selection of the appropriate resin greatly enhances the performance of such hooks. More specifically I have found that a hook produced with a displacement volume, discussed more fully below, of less than 6xc3x9710xe2x88x926 cubic inches and preferably a displacement volume of less than 4xc3x9710xe2x88x926 cubic inches will provide unusual and outstanding performance with a loop of the lowest loop configuration. Displacement volume, as defined herein, is the volume of a rectangular parallelepiped which delineate the volume of loop displaced when a hook penetrates into the loop to just the point where loops may start to fall into the cavity at the inside of the crook of a hook, as will be more fully appreciated from the description below.